This invention relates to receivers for frequency shift keyed signals and more particularly to digital devices for asynchronous demodulation of such signals.
Frequency shift keyed (FSK) modulation is employed to serially transmit digital data over a telephone line or similar media. The FSK signal is produced by shifting a carrier signal between two distinct frequencies, commonly referred to as mark and space tones. The value of each digital data bit determines the frequency of the carrier to be transmitted. Zero bits generate the space tone and one bits produce the mark tone. Each bit is sent by shifting the carrier to the corresponding frequency for a fixed period of time referred to as the bit interval.
The reception of FSK signals must include a mechanism for demodulating, or decoding, the signal to recover the binary data. The reception involves the detection of unknown signals having an unknown phase. Prior art systems for the demodulation of FSK signals have generally employed analog filtering approaches or alternatively, phase or frequency locked decoding techniques. In the first approach, the signal to be demodulated is applied to a pair of analog filters, each of which has a pass band centered at the mark and space tones of the FSK signal being received. The filters must have a well-defined yet relatively narrow pass band and detection is based upon sensing the output of each filter. The filter having the greatest output indicates the reception of the corresponding character. Often the design of such analog filters is extremely laborious and must include complex signal conditioning. The resulting demoduator apparatus is expensive to fabricate and does not readily lend itself to fabrication on a single integrated circuit using conventional techniques. Furthermore such apparatus is subject to frequency drift and precise demodulation of the incoming information can not be reliably achieved over extended periods of time or through variations in temperature without periodic readjustment. Such adjustment is not practical when the circuitry is incorporated in an integrated circuit.
Similarly, when phase or frequency locked loops are employed for the purpose of demodulating FSK information, highly precise closed loop feedback techniques must be used and the band width of the phase locked loop must be generally wider than the optimum detection band width. Additionally, the resulting analog circuit is generally difficult to design and costly to implement. These circuits too are not well suited for implementation through the use of conventional integrated circuit fabrication techniques.